This invention relates generally to systems for recovering information from information storage media, and, more particularly, to apparatus for centering a beam of radiation on an information track in such a medium.
Tracking apparatus of this particular type are especially adapted for use in a system for recovering information from an optically-readable video disc, wherein video signals and other types of information are stored in a plurality of substantially circular information tracks arranged in either a spiral or concentric circle pattern. Typically, each information track on the disc comprises a succession of light-reflecting and light-scattering regions representative of a high-frequency carrier that is frequency modulated by the video signal being stored. The successive light-scattering regions typically take the form of bumps or pits which are separated by a planar reflective area, such bumps or pits having sloped sides adapted to scatter any incident radiation in predetermined directions.
A video disc player ordinarily operates to recover a video signal from such a video disc by focusing a beam of radiation onto the disc with a microscope objective lens, while the disc is rotated with respect to the beam at a prescribed angular velocity. Radiation reflected from the disc will have an intensity that is modulated by the successive light-reflecting and light-scattering regions, and will be transmitted back through the objective lens to a photodetector for detecting the modulated radiation. In order to store a reasonably large amount of information on the disc, successive information tracks must be so closely spaced that even a slight degree of disc eccentricity will cause the beam of radiation to traverse a number of adjacent tracks on each revolution. Consequently, some means must be provided for applying transverse or radial corrections to the beam position, so that it substantially follows along the center of the selected information track.
One prior technique for centering a beam of radiation on a track of a video disc utilizes a pair of contiguous photodetectors for detecting radiation reflected from the disc, one such detector adapted to receive radiation reflected from the outboard half of the track and the other adapted to receive radiation from the inboard half of the track. A summation of the signals produced by the two photodetectors produces an information signal representative of the frequency modulated carrier, and a differencing of the two signals produces an error signal indicative of the deviation of the incident beam relative to the center of the track. Coupling this error signal to a movable mirror that directs the incident beam of radiation onto the disc permits the relative radial position of the beam to be controllably adjusted, to bring it into proper alignment with the center of the track. As a result, the information signal will be properly representative of the information recorded on the disc. Although this technique has proved satisfactory for many applications, it requires a precise alignment of the two photodetectors and it is highly sensitive to noise and vibration. Moreover, since the photodetectors must be spaced a substantial distance from the focal point of the reflected radiation, it can sometimes be difficult to produce detectors having sufficient size to receive all of the radiation, yet still having a sufficiently fast response time to detect the frequency modulated carrier.
Another technique for tracking the center of an information track utilizes three separate beams of radiation focused onto the disc by an objective lens, one beam for impinging on the center of the track being read, and two side beams for impinging on opposite edges of the track. The three beams of radiation are reflected from the disc in accordance with the recorded information, and are transmitted back through the objective lens to three separate photodetectors. A comparison of the signals produced by the photodetectors for the two side beams produces an error signal for controlling the angular position of a movable radial tracking mirror that directs the three beams onto the disc. Thus, the center beam can be moved radially into alignment with the center of the track. Although this technique, like the technique described earlier, is satisfactory for use in many applications, it requires the use of three separate beams of radiation and it requires a precise alignment of the beams with respect to the corresponding photodetectors and with respect to the track.
Accordingly, it will be appreciated that there has been a need for an improved center tracking apparatus that does not require an unduly precise alignment of certain of its elements and that is not unduly sensitive to noise and vibration. The present invention meets this need.